Methods have been described for preparing sheet materials by flocculating and dewatering mixtures of various organic and inorganic fibers and vermiculite that has been chemically treated to effect swelling of the vermiculite lamellae prior to the application of a shearing force. For example, U.S. Pat. No. 3,434,917 describes a method for conversion of vermiculite ore into a pulp suitable for conversion into sheet form by conventional papermaking methods. The ore is treated with sodium chloride and a lithium salt solution before shearing, and asbestos or cellulose can be added to the vermiculite suspension before flocculating with potassium hydroxide or hydrochloric acid. U.S. Pat. No. 3,654,073 discloses a method for making a nonburning paper from a mixture of asbestos, glass fibers, lithium-exchanged vermiculite and a latex resin using an inorganic flocculating agent. A flocced mineral material that is water resistant and high temperature resistant is described in U.S. Pat. No. 4,707,298. The material comprises a swelled layer silicate gel prepared from vermiculite and/or mica and may optionally be mixed with fibers. A flocced mineral material that comprises a binder and a swelled layer silicate gel is disclosed in U.S. Pat. No. 4,762,643. The material may optionally contain a fibrous material selected from silicon, boron, carbon, ceramic, glass and metal fibers. However, materials made from chemically delaminated vermiculite can be difficult to drain and form into a continuous sheet on standard papermaking equipment.
A process for the production of an aqueous dispersion of mechanically delaminated vermiculite ore particles is disclosed in U.S. Pat. No. 4,801,403. This process for producing vermiculite dispersions is less costly and time consuming than chemical delamination, since it does not involve the steps of treatment of the ore with various salt solutions to effect cation exchange, washing out excess salts and swelling the ore prior to shearing. It also avoids the cost of maintaining the high processing temperatures, typically greater than 500.degree. C., required for thermal exfoliation, another known method for expanding vermiculite prior to mechanical shearing. Also disclosed in U.S. Pat. No. 4,801,403 is a process for the production of solid articles by depositing a dispersion of mechanically delaminated vermiculite lamellae on a surface and removing water from the vermiculite. Fibers are not included in the dispersion, nor is a method disclosed for making composite sheets from such a dispersion using papermaking techniques.